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2018.04.04

# 3. SPECTRE VS BITCOIN – 概述

SPECTRE adopts many of Bitcoin’s solution features. In particular, miners create blocks, which are batches of transactions. A valid block must contain a solution to the PoW puzzle (Bitcoin for example, uses PoW that is based on partial SHA256 collisions). The block creation rate, denoted λ, is kept constant by the protocol by occasional readjustments of the PoW difﬁculty; we elaborate on this mechanism in SPECTRE in Appendix D. The size of a block is limited by some B KB.

SPECTRE采用了比特币的许多解决方案的功能。 特别是，矿工创建区块，即成批的交易。 一个有效的区块必须包含PoW难题的解（以比特币为例，使用基于部分SHA256冲突的PoW）。 (译注： 这里的部分冲突指的生成的散列值不要求每一位都一样，只要部分一样就可以了) 出块率，记为λ，通过协议定期重新调整PoW难度而保持不变; 我们在附录D中详细说明了SPECTRE的这种机制。 块的大小限制为某个值B KB。

Bitcoin’s throughput can be increased by increasing either the block size limit (which in turn increases $D$) or\and the block creation rate $\lambda$. Alas, it is well established that the security threshold of Nakamoto Consensus deteriorates as $D \cdot \lambda$ increases:

### 定理 2. [比特币是不可扩展的] 比特币协议的安全阈值随着D·λ增加而变为零。

The proof of this theorem appears in various forms in previous works, see [18], [15], [7]. To maintain a high security threshold, Bitcoin suppresses its throughput by keeping λ low – 1/600 blocks per second. This large safety margin is needed because λ (and B ) are decided once and for all at the inception of the protocol. Consequently, even when the network is healthy and D is low, Bitcoin suffers from a low throughput – 3 to 7 transactions per second, and slow conﬁrmation times – tens of minutes. In contrast, SPECTRE’s throughput can be increased without deteriorating the security threshold:

### 定理 3. [SPECTRE 是可扩展的] 对于任何D·λ，SPECTRE的安全阈值为50％。

Therefore, in the context of the Distributed Algorithms literature, SPECTRE falls into the partial synchronous setup, as it remains secure for any value of D. Theorem 3 is proven rigorously in Appendix E.

Of course, λ cannot be increased indeﬁnitely or otherwise the network will be ﬂooded with messages (blocks) and become congested. Theorem 3 “lives” in the theoretical framework (speciﬁed in Section 2), which does not model the limits on nodes’ bandwidth and network capacity. Practically, these barriers allow for a throughput of thousands of transactions per second, by setting λ = 10 and b = 100, for instance. For further discussion refer to Appendices B and D.

Asymptotically, SPECTRE’s conﬁrmation times are in $\mathcal{O}(\frac{ln(1/ϵ)}{λ(1-2α)}+\frac{D}{1-2α})$ . In practice, this allows for conﬁrmation times of mere seconds, under normal network conditions. When running RobustTxO, each node in SPECTRE uses its own upper bound on the recent D in the network. This bound affects only its own operation—underestimating D will result in premature acceptance of transactions, and overestimating it by far will delay acceptance unnecessarily (by a time linear in the difference). Importantly, in case of network hiccups and long network delays, the node can switch in his local client to a more conservative bound on D without coordinating this with other nodes.

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